1. Field of the Invention
The present invention relates to amplification of ultrashort-pulses, and more specifically, it relates to means for stretching ultra-short pulses prior to amplification.
2. Description of Related Art
In recent years, amplification of ultrashort optical pulses using the technique of chirped-pulse amplification (CPA) has become commonplace. With this technique, high energy, ultrashort pulses are made possible by first stretching the pulse in time, amplifying it, and finally recompressing it temporally. This technique has made possible the production of terawatt and now even petawatt class femtosecond lasers (see M. D. Perry, and Gerard Mourou, "Terawatt to Petawatt Subpicosecond Lasers," Science, Vol. 264, pp. 917-924 (May 13, 1994)). In these CPA laser systems, it is required to stretch ultrashort-pulses (or pulses having sufficient bandwidth to become ultrashort) prior to amplification. An ultrashort-pulse is one having a duration in the range of 5 femtoseconds to 50 picoseconds. Typically the pulse is stretched several thousand times to a duration typically greater than 100 picoseconds prior to injection into the amplifier system. Following the stretcher, the pulse is then amplified to high energies and recompressed to optimally its original temporal length.
A key element in such systems is the pulse stretcher (or pulse expander) which, in early designs, was composed of a pair of diffraction gratings separated by a 1-to-1 telescope (see O. E. Martinez, IEEE Journal of Quantum Electronics, Vol. QE-23 No. 1, pp. 59-64 January 1987), and M. Pessot, P. Maine, and G. Mourou, "1000 Times Expansion/Compression Of Optical Pulses For Chirped-Pulse Amplification," Optics Communications, Vol. 62 No. 6, 419-422 Jun. 15, 1987)). However, due to a sensitivity to grating alignment, these double grating designs were replaced with a folded design using a single grating and lens with a mirror at the Fourier plane (see M. D. Perry and F. G. Patterson, "Design and Performance of a 10 Tw Nd:Glass Laser System Based on Chirped Pulse Amplification," Conference on Lasers and Electrooptics, Baltimore, Md., May 1989). These early pulse stretchers (and numerous derivatives of these basic designs, e.g., J. Weston, et al, "Laser Pulse Stretcher and Compressor With Single Parameter Wavelength Tunability," U.S. Pat. No. 5,349,591) used refractive optics (lenses) and performed well for most CPA systems where the final pulse width was greater than approximately 100 femtoseconds. As CPA systems continued to evolve to shorter pulse durations, however, the large spectral bandwidths and large stretching ratios involved demanded a change from these stretcher designs due to chromatic aberration in the refractive optics.
Chromatic aberration in these systems limits the stretching ratio achievable and the duration and contrast of the recompressed pulse. In most CPA systems, it is the pulse stretcher which dominates the chromatic aberration. Chromatic aberration is most easily minimized by eliminating the use of refractive components (e.g., lenses) in the optical design of the pulse stretcher. This has led to the development of numerous all-reflective designs or designs which employ achromatic lenses. These designs improve, but do not eliminate, the chromatic aberration. This is a result of the fact that although the focusing mirror (or lens) may be achromatic, the stretcher as an optical device is not due to the dispersion of the grating. Furthermore, other optical aberrations such as spherical aberration, coma, astigmatism and vignetting are present in stretcher designs. (There are numerous analyses of the effects of aberrations on stretcher performance in the literature. One good (but highly technical) example is C. Fiorini, et al, "Temporal Aberrations Due to Misalignments of a Stretcher-Compressor System," IEEE Journal of Quantum Electronics Vol. 30 No. 7, pp. 1662-1670 (July 1994)). These aberrations can be as detrimental to the performance of the laser system as chromatic aberration.
The diffraction grating employed in the stretcher disperses the various wavelength components in the laser pulse along different angles (this is the key to pulse stretching in the first place). The chromatic aberration of the pulse stretcher, and in fact, the overall performance of the laser system itself, is determined by the ability of the stretcher to propagate the different frequency components along widely varying paths within the stretcher and reassemble them into a well collimated beam with minimal phase aberration and no spatial chirp. Spatial chirp refers to the case where a broad-bandwidth laser beam has a non-uniform distribution of frequency-components across the beam cross section.
In order to minimize chromatic aberration, several all-reflective stretcher designs have been developed (see B. E. Lemoff and. C. P. J. Barty, Optics Letters, Vol. 18 No. 19 pp. 1651-1653 (Oct. 1, 1993), and J. V. Rudd, et al, Optics Letters, Vol. 18 No. 23, pp. 2044-2046 (Dec. 1, 1993)). These designs either employ large special optics, are limited in the stretching ratio (typically to less than 400 psec), are used off-axis, and involve a large number of elements (see B. E. Lemoff and C. P. J. Barty, Optics Letters, Vol. 18, p. 1651 (1993)) which complicate alignment. As mentioned previously, these designs are aberration-free only in regards to the lack of chromatic aberration in the reflective mirrors. They still contain the other optical aberrations. The only stretcher design in which all optical aberrations up to third-order are indeed zero (for a monochromatic beam) are those based on the Offner optical triplet (see A. Offner, U.S. Pat. No. 3,748,015 (1973)). These stretcher designs (see D. Du, et al, "Terawatt Ti:Sapphire Laser with a Spherical Reflective-Optic Pulse Expander," Optics Letters, Vol. 20 No. 20, pp. 2114-2116 (Oct. 15, 1995), and G. Cheriaux, et al, "Aberration-Free Stretcher Design for Ultrashort-Pulse Amplification," Optics Letters, Vol. 21 No. 6, pp. 414-416 (Mar. 15, 1996)) provide an optically aberration-free unit magnification telescope within the stretcher and as such represent the most advanced current art. These Offner-based designs require a pair of extremely high optical quality, matched curved mirrors and exhibit extreme sensitivity to alignment.
To date, the stretchers employing either achromatic lenses or reflective elements (parabolic or spherical mirrors) suffer from one or all of the following: they (i) still contain significant chromatic and/or optical aberration as a result of off-axis use of the focusing element, (ii) provide low stretching ratios, (iii) are extremely complex involving a number of elements, (iv) exhibit extreme sensitivity to alignment and/or (v) employ very expensive aberration corrected optical elements.